1. Field of the Invention
The present invention relates to a radio receiver, and more specifically to the configuration of an RF front-end receiving unit of a radio receiver used in a MIMO system or a SIMO system.
2. Description of the Related Art
Wireless communication systems have grown rapidly because of advantages such as mobility, flexibility, and inexpensive network configuration. In wireless systems, there are three major impairments associated with radio channels: fading, delay spread, and co-channel interference. In order to achieve high-speed, high-quality and high-capacity communications, countermeasures should be employed to combat these impairments. MIMO (multiple-input multiple-output) technology has drawn increasing attention since it can provide radio channels capable of transferring information in parallel within a given bandwidth and significantly increase the attainable capacity. It is currently being used for third-generation cellular systems (W-CDMA) and its adoption for future high-performance modes of the highly successful IEEE 802.11 standard for wireless local networks (WLAN) and 3.5-generation cellular system such as High Speed Downlink Packet Access (HSDPA) is under discussion.
MIMO-related topics also occupy a considerable part of today's academic communications research. MIMO wireless systems are those that have multiple antenna elements at both the transmitter and receiver. The extra degrees of freedom afforded by the multiple antennas can be used for increasing bit rates in two different ways. One is the creation of a highly effective antenna diversity system; the other is the use of the multiple antennas for the transmission by several parallel data systems to increase the capacity of the system.
FIG. 1 shows the configuration of the MIMO system in a conventional wireless system. The MIMO system shown in FIG. 1 has M transmitter antennas and N receiver antennas 11, 12, 13 . . . 1N. M different data streams are transmitted from M transmitter antennas in parallel, and received by N receiver antennas in parallel (simultaneously). The received RF signals are downconverted to baseband signals (in-phase signal element and quadrature-phase signal element) in N front-end circuits (downconverters) 21, 22, 23 . . . 2N provided corresponding to N array antennas. Then, since the output of the in-phase signal elements of the downconverters pass through low-pass filters (LPF) 31, 32, 33 . . . 3N, they are filtered and simultaneously their waveforms are restored to the original state. Similarly, since the output of the quadrature-phase signal elements of the downconverters pass through low-pass filter (LPF) 41, 42, 43 . . . 4N, they are filtered and simultaneously their waveforms are restored to the original state. The filtered signals are converted to digital signals in 2N A/D converters (ADC) 51, 52, 53 . . . 5N, and 61, 62, 63 . . . 6N after passing the LPFs.
The above-mentioned MIMO system is disclosed by, for example, the following patent literature 1 and 2.
[Patent Literature 1] U.S. Patent Document No. 6,252,548 “Transceiver Arrangement for A Smart Antenna System in A Mobile Communication Base Station”
[Patent Literature 2] U.S. Patent Document No. 6,728,517 “Multiple-Input Multiple-Output Radio Transceiver”
In the conventional MIMO system, when a receiver antenna is expanded, a newly added unit is provided with the component of an RF front-end unit, for example, a downconverter, and this causes increased complexity of the system configuration, higher power consumption, higher fabrication costs, expansion of the system configuration, and increase in related cabling requirements, thus making physical configuration of the system difficult. The rapid growth of the wireless communication market has created the need for low cost, compact, and low power circuits. However, the conventional MIMO system cannot support the above-mentioned demand, and further the conventional SIMO (Single-Input Multiple-Output) system has similar problems.